Piezoelectric devices are attractive candidates as control valve actuators in common rail fuel injectors for diesel engines. The precise longitudinal deflection characteristic of piezoelectric devices in conjunction with their rapid dynamic response provides the potential of achieving meaningful control over the rate of fuel injection. Additionally, the relative high load capability of piezoelectric devices is consistent with the extremely high pressure environment of common rail fuel injectors.
Unfortunately, piezoelectric devices suffer from an extremely small deflection capability. Furthermore, piezoelectric devices are made from materials that exhibit a coefficient of thermal expansion that is much lower than the iron-based materials commonly used to house the piezoelectric devices within the fuel injectors. Accordingly, piezoelectric devices exhibit thermally induced lash that is significantly greater than their deflection capability. As a result, piezoelectric devices are rendered unusable as an actuator for fuel injectors without a means for thermal expansion compensation.
A hydraulic lash adjuster has been considered as a means for achieving thermal expansion compensation in a piezoelectric actuated fuel injector. A conventional hydraulic lash adjuster typically uses a relatively large volume liquid filled working chamber to compensate between the actuated and the actuating members. However, due to the extremely high pressures encountered in common rail fuel injectors, these conventional hydraulic lash adjusters will experience a loss in length caused by compression of the liquid. Since the piezoelectric actuator has a very short stroke, this length loss makes the conventional hydraulic lash adjuster unusable as a means for thermal expansion compensation in a piezoelectric actuated fuel injector.
Therefore, it is desirable to provide a hydraulic lash adjuster having a sufficiently small internal working volume to properly compensate for the length differences of the piezoelectric actuated fuel injector.